FLAG-tag, or FLAG octapeptide, is a polypeptide protein tag that can be added to a protein using recombinant DNA technology. It can be used for affinity chromatography, then used to separate recombinant, overexpressed protein from wild-type protein expressed by the host organism. It can also be used in the isolation of protein complexes with multiple subunits.

A FLAG-tag can be used in many different assays that require recognition by an antibody. If there is no antibody against the studied protein, adding a FLAG-tag to this protein allows one to follow the protein with an antibody against the FLAG sequence. Examples are cellular localization studies by immunofluorescence or detection by SDS PAGE protein electrophoresis.

The peptide sequence of the FLAG-tag from the N-terminus to the C-terminus is: DYKDDDDK (1012 Da). It can be used in conjunction with other affinity tags, for example a polyhistidine tag (His-tag), HA-tag or Myc-tag. It can be fused to the C-terminus or the N-terminus of a protein. Some commercially available antibodies (e.g., M1/4E11) recognize the epitope only when it is present at the N-terminus. However, other available antibodies (e.g., M2) are position-insensitive.

Receptor expressed in lymphoid tissues (RELT), also known as tumor necrosis factor receptor superfamily, member 19-like (TNFRSF19L), is a member of the TNF-receptor superfamily. This receptor is especially abundant in hematologic tissues. It has been shown to activate the NF-kappaB pathway and selectively bind TNF receptor-associated factor 1. RELT/TNFRSF19L is capable of stimulating T-cell proliferation in the presence of CD3 signaling, which suggests its regulatory role in immune response. RELT/TNFRSF19L is a type I transmembrane glycoprotein with a cysteine-rich extracellular domain, possessing significant homology to other members of the TNFR superfamily, especially TNFRSF19, DR3, OX40, and LTbeta receptor. RELT/TNFRSF19L is able to activate the NF-kappaB pathway and selectively binds tumor necrosis factor receptor-associated factor 1. RELT/TNFRSF19L is able to activate the NF-κB pathway and selectively binds tumor necrosis factor receptor-associated factor 1. Although the soluble form of RELT fusion protein does not inhibit the one-way mixed lymphocyte reaction, immobilized RELT/TNFRSF19L is capable of costimulating T-cell proliferation in the presence of CD3 signaling.